1. Field of the Invention
The present invention relates to a glass composition suited for producing graded index lenses having a refractive index gradient in their cross sections. More particularly, the invention relates to a mother glass (i.e., homogeneous glass before ion exchange) composition for graded index lenses which is excellent in drawability and ion-exchanging properties.
2. Description of the Related Art
A graded index lens is a rod lens having such a refractive index distribution in its cross section that the refractive index changes from the center toward the periphery. It has many advantages, for example, that it has an image forming action even when both sides thereof are flat and that it can be easily produced as a microfine lens. Because of such advantages, graded index lenses are recently used in optical systems in a wide range of applications such as copiers, facsimiles, LED array printers, and liquid-crystal shutter printers.
The graded index lens, usable in such a wide range of applications, is produced by an ion-exchange process in which a glass body containing a first cation capable of constituting a modifying oxide is brought into contact at a high temperature with a molten salt containing a second cation capable of constituting a modifying oxide to exchange the first cation in the glass body for the second cation in the molten salt and thereby form in the glass body a refractive index distribution in which the refractive index changes from the center of the glass body toward the periphery.
For example, it is known that subjecting a glass body containing cesium to ion exchange in molten potassium nitrate gives a graded index lens having excellent chromatic aberration (see JP-B-51-21594; the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d). However, the angular aperture of this lens is as small as about 10xc2x0 at the most. JP-B-59-41934 discloses a process for obtaining a graded index lens having a greater angular aperture, in which a glass body containing lithium is subjected to ion exchange in molten sodium nitrate. Also known is a mother glass composition capable of giving a graded index lens having an angular aperture of 13xc2x0 or greater (see JP-A-7-88234; the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d).
However, experiments following Examples of JP-B-59-41934 revealed that the angular aperture is about 12xc2x0 at the most and the Abbe""s number, which is an index to dispersion of light, of the composition containing lead oxide is as small as less than 40. Furthermore, the Examples of JP-A-7-88234 indicate that the use of 5 mol % or more lead oxide, which was necessary for obtaining an angular aperture of 13xc2x0 or greater, has made the mother glass show enhanced dispersion and have an Abbe""s number less than 40.
In view of such conventional lenses and with the recent broadening of applications of graded index lenses, there is a desire for a lens showing reduced dispersion, i.e., a large Abbe""s number, and having a large angular aperture.
An object of the invention, which eliminates the problems described above, is to provide a mother glass composition for giving a graded index lens which shows reduced dispersion, is highly suitable for mass-production, and has an angular aperture of 13xc2x0 or greater.
The present inventors made intensive investigations in order to overcome the problems described above and, as a result, have found the following. A glass based on SiO2xe2x80x94TiO2xe2x80x94Li2Oxe2x80x94Na2O and containing no lead oxide can be obtained as a glass having a desired refractive index by incorporating thereinto at least a given amount of one or more ingredients which are selected from oxides of the metal elements ranging from yttrium, having an atomic number of 39, to tantalum, having an atomic number of 73, and excluding the lanthanides and which are less apt to cause glass coloration. Furthermore, this glass can be made less apt to suffer devitrification by incorporating Ta2O5 and ZrO2 in a given proportion and regulating the total amount of Ta2O5, ZrO2, and Na2O to a value not larger than a given amount. In addition, a glass which can readily undergo ion exchange at lower temperatures and is less apt to develop cracks can be obtained by incorporating B2O3 and GeO2 in a total amount not smaller than a given value and thereby lowering the transition point.
The invention provides a mother glass composition for graded index lenses which comprises the following glass components:
40 to 60 mol % SiO2,
2 to 12 mol % TiO2,
2 to 22 mol % MgO,
12 to 20 mol % Li2O,
2 to 13 mol % Na2O,
0 to 10 mol % GeO2,
0 to 10 mol % B2O3, and
0 to 5 mol % Al2O3,
and further comprises:
0 to 5 mol % ZnO,
0 to 7 mol % Y2O3,
0 to 7 mol % Nb2O5,
0 to 7 mol % In2O3,
0 to 7 mol % ZrO2,
0 to 16 mol % BaO,
0 to 5 mol % SnO2, and
0 to 7 mol % Ta2O5,
provided that the total content of ZnO, Y2O3, Nb2O5, and In2O3 is from 0 to 10 mol %.
For inhibiting devitrification while maintaining a large angular aperture, it is desirable to regulate the contents of ZrO2 and Ta2O5 to from 1 to 5 mol % and from 1 to 5 mol %, respectively, and to regulate the ratio of the Ta2O5 content to the ZrO2 content to from 0.2 to 2.5.
For further inhibiting devitrification, it is desirable to regulate the total content of ZrO2, Ta2O5, and Na2O to from 4 to 15 mol %, that of MgO, BaO, and TiO2 to from 10 to 25 mol %, and the ratio of the MgO content to the BaO content to from 0.4 to 2.5.
Furthermore, for enabling ion exchange to be conducted at a lower temperature to facilitate heat treatment, it is desirable to regulate the total content of GeO2 and B2O3 to from 2 to 15 mol %.